In present optical core networks, the optical transport network (OTN) has been widely used as basic platforms. The OTN is optical transport standards recommended by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) in 2000. As illustrated in FIG. 1, in an OTN optical core network 1, network devices 2 are connected to each other via an optical fiber network. The OTN optical core network 1 enables high-capacity transmission by accommodating or multiplexing, in OTN frames, various client signals such as synchronous optical network (SONET) signals, Gigabit Ethernet (GbE, trademark) signals, and fiber channel (FC) signals. The client signals are mapped into OTN frames in the order of optical channel payload unit (OPU) frames, optical channel date unit (ODU) frames, and optical transport unit (OTU) frames. When the client signals are mapped into the OPU frames, stuff control is performed using the generic mapping procedure (GMP) which is an asynchronous accommodation scheme (see Japanese Laid-Open Patent Publications Nos. 2010-212890 and 2012-023647, for example).
FIG. 2A illustrates a configuration example of a general network device 2. Optical modules 211 to 21k receive multiple client signals (optical signals). The received signals are subjected to data reception processing by low-speed interface processing sections 221 to 22k, and then the data are selected by a cross-connect section 23. The selected data are multiplexed by a multiplexer 24, and then accommodated in OTN frames by a high-speed interface processing section 26. The OTN frames are converted into optical signals by an optical module 27, and the optical signals are transmitted to an OTN network. OTN frames received from the OTN network are subjected to the above processes in the reverse order and output from the optical modules 211 to 21k as client signals.
FIG. 2B illustrates a circuit configuration of each low-speed interface processing section 22. A serializer and deserializer (hereinafter referred to as “SERDES”) 31 is configured to use a reference clock to detect a change point from a serially transmitted client signal and reproduce a recovery clock synchronous with the client signal. The SERDES 31 is also configured to perform serial-to-parallel conversion on the client signal and output parallel data.
In present circumstances, the SERDES 31 is capable of reproducing a recovery clock synchronous with a client signal if the client signal is at a rate of 600 Mbps or higher.
Since the speed of the SERDES 31 has been increasing, the SERDES is incapable of reproducing a recovery clock synchronous with a client signal if the signal is a low-speed signal (155.52 Mbps or lower).
For this reason, digital oversampling using the high-speed SERDES 31 is employed to receive a low-speed signal such as an optical carrier-level 3 (OC-3) signal.
The SERDES 31 samples a serially transmitted client signal at a rate of 8 times the original rate, for example, performs serial-to-parallel conversion on the signal, and outputs parallel data.
An oversampling extraction circuit 32 is configured to detect a change point in the parallel data output from the SERDES 31, extract data from a midpoint between one data change point and a subsequent data change point as valid data, and output the extracted data together with an oversampling extraction data valid signal.
If no data change point exists, the circuit extracts valid data using the position of the previous change point as a reference.
A stuff operation section 33 is configured to compute, according to the type of a client signal, the number of data of the client signal received per OPU frame (Cn value), the number of data of the client signal to be actually mapped into one OPU frame (Cm value), and the number of data of the client signal to be accumulated without being mapped (ΣCnD value).
The Cn value is a theoretical value representing the amount of data to be mapped into one OPU frame in units of n bits, and n=1 in the case of OC-3 (mapping processing on a per-bit basis).
When a recovery clock and the received client signal are synchronous, the Cn value may be obtained from the number of edges of the recovery clock per OPU frame. However, because oversampling is performed in the configuration of FIG. 2B, the recovery clock is asynchronous with the received client signal.
Hence, the Cn value is computed from the oversampling extraction data valid signal.
An OPU frame generation section 34 is configured to generate an OPU frame, perform stuff control based on the Cm value obtained by the stuff operation section 33, and map the client signal into the OPU frame.
The OPU frame generation section 34 is also configured to insert the Cm value and the ΣCnD value into the overhead of the OPU frame as justification control (JC) bytes.
An OPU frame format and a method of mapping a client signal into an OPU frame are defined in the OTN optical transport standard.
When a low-speed client signal is oversampled and output as parallel data using a reference clock, a gap (or phase deviation) between the frequency of the reference clock and the frequency of the client signal is accumulated gradually if the reference clock and the client signal are asynchronous.
This causes a gap corresponding to one bit of the client signal at a fixed cycle according to the difference between the frequencies of the reference clock and the client signal, as will be described later.
This gap corresponding to one bit of the client signal may cause jitter and wander in the stuff control.
In addition, the gap corresponding to one bit of the client signal is also propagated to the stuff operation section 33 and the OPU frame generation section 34.
Since the OPU frame reception side reproduces the clock of the client signal based on the received Cm value and ΣCnD value, the client signal extracted by the reception side has a gap corresponding to one bit (6 ns in the case of OC-3) from the original signal.
This gap causes jitter and wander on the reception side.
The embodiments aim to provide an optical transmission device and a signal frame generation method which may reduce a gap caused when a low-speed client signal is mapped into an OPU frame and thereby suppress jitter and wander.